The preparation and characterization of a murine monoclonal antibody (McAb 33.1) has been completed. The tissue and cellular distribution of the 33.1 antigen suggests that it is a polymorphic human B-lymphocyte specific marker. Flow cytometric analysis of a panel of EBV transformed human homozygous typing cells suggest that it co-segregates with MB1 and MB3 positive cells. Limited radioamino acid sequence studies suggest that it is a new non-DR, but Ia-like antigen.